Mordant Calculator
Mordant weight from fiber weight and type (alum, iron, tannin).
About this calculator
Mordants are the chemical bridge between a natural dye and the fiber — without one, most plant-based dyes wash or fade out quickly because they can't bond directly to cellulose or protein. This calculator applies your mordant percentage (% OWF, on the dry weight of fiber) directly to fiber weight to get the grams needed per bath, and does the same for an optional assistant mordant like cream of tartar, which is commonly used alongside alum on protein fibers (wool, silk) to brighten and soften the resulting shade. It separately computes the mordant bath's water volume from your fiber weight and chosen liquor ratio, then derives the mordant's concentration in the bath (grams per liter) by dividing the per-bath mordant weight by that volume — a useful number for comparing against published mordant-bath recipes.
If you set more than one mordant bath, the calculator multiplies both mordant weight and water volume by the bath count to give running totals, reflecting techniques like cellulose fibers that get a tannin pre-mordant followed by a separate alum bath. The percentages baked into the reference notes vary a lot by mordant type and fiber: alum runs 12-20% for protein fibers but needs more (15-25%) on cellulose since plant fibers bind metal mordants less readily, iron and copper are used sparingly (2-4%) because they actively shift and darken color rather than just fixing it, and using too much iron in particular can weaken and embrittle fiber over time — so more is not simply "more mordanted," it can also mean a different final color and a more fragile textile.
Inputs
Results
Mordant per bath (g)
75
How to Use This Calculator
- Enter Fiber weight (g), Mordant (% OWF), and Assistant mordant (% OWF).
- Set Liquor ratio (X:1) and Number of mordant baths.
- Review the Mordant per bath (g) result.
- Use Assistant per bath (g) and Bath volume (L) to inform your decision.
How the result changes with Fiber weight (g)
| Fiber weight (g) | Mordant per bath (g) |
|---|---|
| 250 | 37.5 |
| 375 | 56.25 |
| 750 | 112.5 |
| 1,250 | 187.5 |
What each input means
- Fiber weight (g)
- Dry weight of fiber or fabric to mordant, in grams.
- Mordant (% OWF)
- Mordant dosage as % on weight of fiber. Alum: 12-20%, iron: 2-4%, tannin: 8-12%.
- Assistant mordant (% OWF)
- Cream of tartar or other assistant. Use 6% with alum on protein fibers; 0 if not needed.
- Liquor ratio (X:1)
- Water-to-fiber ratio. 20:1 typical for mordanting in a pot.
- Number of mordant baths
- Some fibers benefit from multiple mordant baths (e.g., cellulose with tannin + alum).
What each result means
- Mordant per bath (g)
- Grams of mordant needed per bath.
- Assistant per bath (g)
- Grams of assistant mordant (cream of tartar) per bath.
- Bath volume (L)
- Water volume per mordant bath.
- Total water (L)
- Total water across all mordant baths.
- Mordant concentration (g/L)
- Mordant concentration in the bath.
- Total mordant all baths (g)
- Total mordant if re-dosing for each bath.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFiber weight (g) = 500, Mordant (% OWF) = 15, Assistant mordant (% OWF) = 6, Liquor ratio (X:1) = 20 = 5 input(s) provided
- Calculate Mordant per bathMordant per bath = fiberWeightG * (mordantPctOwf / 100)75 = 75
- Calculate Assistant per bathAssistant per bath = fiberWeightG * (assistantPctOwf / 100)30 = 30
- Calculate Bath volumeBath volume = (fiberWeightG / 1000) * liquorRatio10 = 10
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator recommend a higher alum percentage for cellulose fibers than for protein fibers?
Alum's default help text lists 12-20% OWF for protein fibers like wool and silk versus 15-25% for cellulose fibers like cotton and linen. Cellulose fibers bind metal-salt mordants like alum less readily than protein fibers do, so more of the mordant has to be applied to fix a comparable amount to the fiber surface.
What does the assistant mordant field (cream of tartar) actually do?
It's calculated the same way as the primary mordant — fiberWeightG × (assistantPctOwf / 100) — and is commonly dosed at 6% OWF alongside alum specifically on protein fibers. Cream of tartar brightens and softens the shade alum alone would produce and helps the alum bond more evenly, which is why the calculator keeps it as a separate, optional line rather than folding it into the main mordant percentage.
Why does setting Number of mordant baths to 2 double both the mordant and water totals?
The calculator multiplies both the per-bath mordant weight and the per-bath water volume by numBaths to get totalMordantG and totalWaterL, on the assumption that each bath is prepared fresh rather than reused. This models real techniques like giving cellulose fiber a separate tannin pre-mordant bath followed by a fresh alum bath, each needing its own full dose.
Is it safe to just increase the mordant percentage for a stronger effect?
No — more mordant doesn't just mean "more fixed," it can shift the color outcome and, in the case of iron and copper mordants (used sparingly at only 2-4% OWF), actively saddens or darkens the dye color rather than simply improving fastness. Excess iron in particular can weaken and embrittle the fiber over repeated use, so it's worth staying within the ranges the calculator's helpText suggests for your specific mordant type.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
More in Crafts, Hobbies & Games.